1. Field of the Invention
The present invention pertains to the field of signal processors. More particularly, the present invention pertains to the field of electroacoustic transducers, such as microphones, and the amplification of their output signals. Even more specifically, the present invention pertains to the field of automatic control of the output signal level of electroacoustic transducers.
2. Description of the Prior Art
In the field of electronic audio devices, the use of microphones usually includes microphone pre-amplifiers which are used to boost up the low output voltage of the microphone to a higher operating level. A problem occurs when the dynamic range of the microphone output exceeds the dynamic range of the associated pre-amplifier, a case which commonly occurs. For example, if the sound level near the microphone should exceed a certain maximum level, the output voltage of the microphone may remain undistorted but will be high enough to overload the pre-amplifier causing distortion of the pre-amplifier output signal. A typical case where this problem occurs is in live sound reinforcement where a performer may sing or speak very loudly, causing distortion. It would be desirable to have means to automatically prevent pre-amplifier overload whenever the microphone output would exceed the maximum input level of the pre-amplifier.
In prior art, there are mainly two methods used to deal with this problem. First, an operator can ride gain on the pre-amplifier and manually readjust the pre-amplifier gain to avoid overload as necessary in reaction to ongoing events. The principal disadvantage of this method is the need for a skilled operator dedicated to the duty.
A second approach to deal with the problem is to configure the pre-amplifier into two stages. In this case it is desirable to have a first stage with a relatively low gain first stage, which allows more input headroom. The second stage adds the required gain boost to make up for the reduced gain of the first stage, but also includes an automatic gain control (AGC) limiter function with a preselected threshold level to prevent overdriving subsequent circuitry. Whenever the microphone output signal rises above the threshold level, the limiter's gain is automatically reduced proportionately. In this way the pre-amplifier is prevented from clipping the output signal. This method is effective until the input stage finally reaches an overload level above which the limiter function can no longer prevent any overload distortion. In the sound studio and live recording sessions, the first and second stages just described are usually comprised of separate and independent units. The stages usually involve a separate microphone pre-amplifier stage, set to a relatively low gain and a separate adjustable limiter stage having the necessary make-up gain. A few microphone pre-amplifiers are commercially available which integrate the two stages. The principal disadvantage of this two-stage approach is a significant noise floor penalty, that is the thermal noise is amplified along with the signal, roughly on the order of the net input headroom improvement which is gained. It would be desirable to have means to extend the input dynamic range of a microphone pre-amplifier, through the use of some sort of limiter function, which does not cause a noise floor penalty.
At this point it may be useful to mention that it seems possible to construct a microphone pre-amplifier having electronically variable front-end gam which could be operated with an AGC limiter function, thus avoiding the need for a second stage limiter and the consequential noise floor penalty. However, this type of "automatically controllable" microphone pre-amplifier which may retain the high quality attributes of conventional top-grade pre-amplifiers has not yet been produced and seems unlikely to be practical with presently known technology.